Method and apparatus for efficiently sterilizing air in ducts



April 14, 1942. E. G. F. ARNoTT METHOD lAND A PPARATUS FOR EFFICIENTLYSTERILIZING AIR IN DUCTS 9876s 4 v5 z 2...

ma N w... @Wam E APlil141942- E. G. F. ARNOTTV 2,279,810

METHOD AND APJARATUSl FOR EFFICIENTLY SEIRILIZIIJG' AIR N DUCTS JIIJw//20 l l .April 14, 1942. E Q F, ARNQTT 2,279,810 METHOD AND APPARATUS FQREFFICIENTLY STERILIZING AIR .1N Duccrs Filed' June' 2g, 1940 :5sheets-sheet 5 a 20al or f 600 aaa ./aaa l/200 (loro irak/anar INVENToRtarife/V077' f ATTORNEY Patented Apr. 14, 1942 METHOD AND APPARATUS FoaEFFICIENT- LY sTERIuzING Am m DUc'rs Edward G. Fl Arnett, Montclair, N.J., assigner to Westinghouse Electric & Manufacturing Company, EastPittsburgh, Pa., a corporation of Pennsylvania Application June 28,1940,V Serial No. 342,934

9 Claims.

This invention relates to the destruction of bacteria in air and, moreparticularly,`to the use of ultra-violet generating means in ducts forsterilizing the air transmitted thereby to an auditorium or other roomor portion of a building.

The principal object of my invention, generally considered, is thedevelopment of calculating charts and a method and apparatus basedthereon, whereby ultra-violet generating lamps are disposed to eicientlysterilize the air in conditioning ducts, so that said air may betransmitted to an auditorium or other portions of a building, afterbeing freed of bacteria and other microorganisms carried thereby.

Another object of my invention is the development of charts which are ofassistance in calculating the spacing of Sterilarnps, or otherultraviolet generating apparatus, in air conditioning ducts, forsubstantially freeing the transmitted air of micro-organisms.

A further object of my invention is the provi-'- sion of charts usefulin calculating the numberV and distribution of sterilizing lamps used inan air conditioning duct, involving the placing at the proposed positionof each lamp, a figure representing the intensity of radiation producedby it at the center of the array.

'A still further object of my invention is the eliicient distribution ofsterilizing lamps in an air conditioning duct, by placing the same in astraight length of duct, distributed over the central half of saidlength, whereby it is possible to Figure 4 is a chart showing therelationship between the length of the array of lamps and the lengthtimes the intensity at the center Aof the array, which for 60%Sterilamps is numeriycally equal to the linear velocity of the airfatthe center of the duct in lfeet per minute.

Figure 5 is a chart showing the relationship' between the number ofSterilampsand the linear velocity, as wellas the flow in cubic feet perminute, when employed in a 75 x30 duct, with the lamps arrayed atdiierent distances.

Figure 6 is a horizontal sectional View of `an installation ofSterilamps positioned in anair` duct in accordance with my invention.

Figure '7 is a vertical sectional Aview alongthe axis of the duct shownin Figure 6. Y It is the function of modern air conditioning systems toprovide roomsv with air that is at the proper temperature and humidity,and practicalk ly'free from dust and dirt. This isgenerally-accomplished by recirculating Vair from theY room 'whileadding a small proportionv of fresh-air throughk the necessary heatingor cooling equipment designed for thepurpose. Air in occupied rooms isat times highly contaminated by bacteria Yand otherl floatingmicro-organisms. One observer has determined that bacteria'oating' inassume that the average intensity over said length is equal to one halfthe value at the center'of the array, said lamps being otherwise equallyspaced and disposed as far apart as possible.

Other objects and advantages of the invention, relating to theparticular arrangement and construction of the various parts, willbecome apparent as the description proceeds.

Referring to the drawings:

Figure 1 is a chart showing how the intensityL of radiation from anultra-violet lamp varies with the distance therefrom, plotted onlogarithmic coordinate paper.

Figure 2 is a chart showing the intensity of the radiation produced atthe center of an array by lamps respectively placed at differentpositions indicated by the gures.

Figure 3 is a chart representing, as an example only, the relationshipbetween the length of the array and the corresponding number of lamps,

for 10" spacing in a 75" X 30 duct, and the intensity of the radiationproduced in clicks per second.

the air may remain in an active state for a considerable time, and thatair-borne pathogenic bacteria may be responsible for the transmission ofcertain respiratory diseases. Y q

Ultra-violet radiation of certain wavelengths'is known tohavebactericidal properties'and-has been proposed for killing air borneorganisms. There are various places where such ultra-violet Aradiations`may be used for this purpose, one being in air conditioning ducts. I'I'he proper installation of lamps in such ducts requires considerablecare. A definite amount of radiation is necessary to kill a givenpercentage of air-borne organisms as they move rapidly in the duct. Theintensity of the radiation is greatest at the surface of the lamp anddecreases vwith increasing distance. The most economical `'use ofradiations in a duct is under conditions in which there is nearly auniform dosage-of the air throughout the cross-section. Lampsy should beinstalled in a straight section of the ductin order to utilize theradiations 'before the' air reaches the lamps and after it passes.Whileithe effect of radiation on organisms diminishes with distance,nevertheless the killing will be proportional to the total amount ofradiation; falling on the bacteria. In many cases a longr straightlength is not always available, but the minimum straight portion should,as determined, be at least twice the length occupied by the lamps.

In order to determine the required number of Sterilamps to be placed inan air duct of a given size, utilizing a given air velocity, tc killapproximately 99% of the bacteriaand other micro-organisms in theair,calculations must vbe made to determine the average intensity of end, isroughly two-thirds that at a corresponding distance in a similar planeat the center.

ultra-violet radiation to which the bacteria are exposed in passingthrough the duct. This-was done by first measuring the intensity of theradi,-

,Sterilamp, that -is a discharge lamp manufac-A tured by theWestinghouse Electric 8:; Manufacturing Company under that trade-mark,vhaving A Thus at the sides of the array at the ends of the lamps, theintensity will also be greater than one half 'that at the center.

- Figure 2 can be used for an array having a smaller number of lamps bysimply omitting those coming outside the desired dimensions.

Alsoy charts for different spacings can be dek'termined in the same wayas Figure 2, and

ygreaterlengths can be determined by simply exation at variousvdistances from one, 30. f 100%' an elongated envelope or tube of glasswhich efciently transmits bactericidal rays, such as Corning 972 hightransmissionl"ultra-violet glass, which is a form of Corex glass.` Suchlamps are-- otherwise constructed as described and claimed inthejJarnesapplication, Serial No. 734,620, iiled July 1 1, 1934.v From thesemeasurements a chart wasmade up as shown in Figure 1. shows in fulllines the; intensity alongalineperfpendcular to thelamp through itscenter, and 4in r dotted lines theiintensityalong a line perpendicularto, the .lamp through one end. These results are the average of velamps, and. .aref recorded in terms of clicks per second, plottedagainst the distance from the lamp. v

This unit of intensitywas used throughout and denes lthe amount ofradiant energy in the bactericidal-range which falls per second onastand.-

' ard tantalum phototube used in a click meter circuit, as describediorexample, in the Rentschler et al. patent, #2,037,925 of April 211936,orin the ,Rentschler et al., application. Serial No. 247,294, ledDecember 22, 1938.

In view of the fact thatzthe radiatien intensity, decreasesfrom center.to end, as shown in Figure y.1,f;it w as decided to have the lamp orlamps ex- `tend'transversegto, the air movement, rather thanlongitudinally, so as to have the ends at thesides of the duct where therate of 4flow was decreased because of turbulence. i This arrangementisalso better from a practical standpoint, as installation isfacilitated. i

[ The nextstep is tok plot anarray of lamps ink rowsand columns atsomedenite distance, such 1`0' apart, `as shown Iin Figure ,2. Thedistance was measured from ,the Acenter of thearray to the axis ofeach-.lamp,:the intensity then read off the curve of ,FigureL and thenumerical. value entered `at the positionof the lamp. -Thearray `coversan area 10Q x 70'!v and contains 8 8 lamps.

!Ihe intensity in the center, that is, as indicated by the number 17.038shown `in Figure 2,1 is the of the radiations received from the, lamps,

represented .by sum of "all the other numbers on Fgure2,`,a s Ysaidnumbers arethe values of the V The vchart tending the calculations asdesired. We now need to know the average intensity `along the center ofthe array which will coincide j withthe axis of the air duct in which itis placed.

Using the arrayy of Figure 2, we nd that the intensity ali-)"each' endis a little more than one-half that at the center. Also, at a point inthe duct ata distance of 40" from the center of the array the intensityis considerably greater than zero.

Therefore; the average intensity over a length of The aboveconsideration would be only for bacteria traveling down the center ofthe duct. It isclear that the average intensity at the sides willgbesomewhat greater than one-half that at the, center ,s and, therefore,bacteria traveling lwith the same velocity asthose at the center wouldvreceive only somewhat more than half `as much energy. At the cornersthis is still fur- -ther; reducedto about one-quarter of that at thecenter; It is-found, however,'that for ducts of the sizeqconsidered, andfora normal range of airelielocity, the flow is softurbulentat the sidewalls that, there the air velocity will' be about one-half that at theVcenter, and in the corners it will ybe 4about one-quarter. This meansthat for the purpose'of calculating the required lin ten'sity and numberof lamps for a vgiven air flow, :onlygthat ofthe center of the duct.need be considered.; j

4It, .hasbeen found that a definite amount of ultra-violet radiation isrequired to kll'a sumcientpercentage of bacteria present in owingair.`Thisrar/nount, as measured on the click meter, 'rhas been found tobe that equivalent'to intensity of each lamp at the center of the-array.v

Ina similar manner a chart could be plotted ffor n y lan ,array havinglamps spaced 5" or some other `distance apart.`

Thejintensity at any along the center of P' the array can befoundbyadding up thenindividual intensities of` all thelamps whichcontribute-at Athat point. Thus the intensity at one'end l of,thearrayis foundto be. alittle greater'. than oneehalf that'at thecenter. `There will bea gradual decrease inffintensityin .moving vfrom-thecenter toan end, or from theicenter to aside,

` tof the array. .l o i From Figure v1 itcar be ,seen that :the intensitinrak plane normal to .thefaxisfof the laniplatfan r .The air ow inaduct is usually given threeclicks. "Thus any point in the air receivingan amount .ofradiation correspondingto three clicks will besuilcientlyfree from bacteria for the-desired purpose; f

1n terms ofnumberof cubicn feetper minute V. We are `interested 'in thelinear speed v in 'feet per minute along the duct. This is givenby the'formula vill/ab, for a duct of `rectangular cross 'sectionwhere `a andb. are the dimensions in feetfof theu cross section. YIf No clicks are`required to kill the bacteria, then they must remainintheirradiatedi'section lof the duct for farjtimeflong; enough to. receivethis amount of ,en'ergyiV` EL' is vthe length rin feet of the arrayoflamps, then fthe, time takenl` by the bacteria tog cross` the; arrayisV t'=L/v minutes. .The amount of energy received in this time is equal4to N=60 nt, where t is in minutes, and n is the ,'diif'erentlampspacing infa 75'! x V30, duct, we

average intensity inclicks per second, as determined from the chart ofFigure 2, for example, as we have seen n is the intensity in the centerfof' the array. Then nLab where L is in inches and n in clicks persecond.

That is, the value of nL is numerically equal to the linear speed of theair in feet per minute. Asfan illustration, consider a duct 30 x 75 incross section, with a flow of 30,000 cubic feet per minute. Assume anarray` of lamps (as in Figures 6 and 7) so that n=l3 for an array, as inFigure 2, but shortened from 100 to 40". leaving only 40 lamps,lL=40 or3.3', then clicks=l.34 clicks. This is less than the required value of 3clicks and would not give the substantially complete killing desired. I,110W- ever, V=3000 cubic feet per minute, we need only three instead offive columns of lamps, since then 1L=10.1l, L=l.6'7 feet, and

clicks. Y

Figure 3 is a chart for calculating the intensity at the center of anarray for 10" spacing of eight rows of lamps in a 75" X 30 duct, fordifferent lengths of array or different numbers of lamps. It will benoted that two curves are necessary, one when there is an even number ofcolumns of lamps, and an other when there is an odd number of columns.For example, with a length, or value of L, of 40, there is an odd numberof lamp columns, that is, five, and we use the curve marked ODD whichgives a value of approximately 13 clicks per second with 40 lamps.

If, for example, we have a length of 50", then there is an even numberof lamp columns with 48 lamps, giving a value of approximately 13.8clicks per second at the center of the array.

Figure 4 is a chart showing how the length in inches compares with thevalue of said length times the intensity at the center of the arraywhich for 60% lamps is numerically equal to the linear velocity in feetper minute. Using this chart we nd that with L equal to 40, the linearvelocity with l0" spacing may be up to about 520 feet per minute,whereas for 15" spacing it must be decreased to about 235 feet perminute, and for 25" spacing it must be decreased to about 85 feet perminute. Similarly, if we have an air flow of 1000 feet per minute, wecan employ an array about 25" long with 5" spacing, about 66" long with10 spacing, or about 126" long with 15" spacing.

Turning now to Figure 5 which shows the number of 30 60% Sterilampswhich should be employed for different air velocities and find that withthe air traveling at 800 feetfper minute, we should'u'se aboutforty-eight Sterilamps if they are spaced 15" from'center to center orflty-threi-'ly Sterilamps' if they Iare spaced l0, oreighty-fourif...they aresp'aced Referring now to Figures-6 'and 7, thereis shown a typical installationof forty-Sterilamps spaced 10" betweencenters in a 75 x 30" duct, and vin accordance with our previouscalculations thevlength of the straight portion of this duct is twicethe length ofthellarnp array, that is whereby in accordance with 'Figure3 the intensity at the center vis about 13 clicks per second, and inaccordance with'Figure 4 or Figure 5 with 40 length,'10" spacing, andforty lamps we may employ an air speed as high as 520 feet per minute. lY

In accordance with the foregoing, it will be seen that in order toarrive at the amount of radiation that must be provided in a given airduct, the problem is, first, to determine the in'- tensity at differentdistances from one Sterilamp and then, find the intensity'at the centerof an array of uniformly spaced lamps. From this we determine an averageintensityrof 'radiation along a duct twice as long as the length of thearray, in order that the radiations from the lamps may be efficientlyemployedk and a readily calculable arrangement provided,to s upplysufcient energy to kill bacteria as they pass therethrough. y

In making practical calculations using the foregoing curves, we 'shouldconsider that four lamps can be' operated on the same transformer andif, for example, the number'of lamps gures'out as thirty-seven for adesired vair velocity,- 'we would probably decide on an installation of-f'crty lamps spaced 10" centerl to center, in a'cl'iamber '75" x 30,that is 5 rows of 8 lamps per row, with a lighted length of 40" or atotal length of 80 treatment duct, as illustrated in Figures 5 and Itshould be pointed out that the calculations and curves producedtherefrom, have been checked by practical tests and found to besufnciently close for use in calculating the number of lamps and lampspacing to be best employed for given installations. If it is desired touse greater duct size and air velocities, `it will be necessary toincrease the siZe of the charts from which to get the necessary data fortables and curves. However, the same principles apply as have beendisclosed in the present application. The question of correct percentageto use is dependent upon tube life, temperature and reection. Thesixty-percent example given is an illustration of how percentage entersinto the equations, and also greatly simplifies the use of the chart ofFigure 4.

Reflection at the walls and absorption by the lamps have not beenconsidered, although it is felt that the former clearly outweighs thelatter if reilecting walls are used. 'I'he Value of three clicksV for Nois considered to be a typical figure, although the form of the functionsfrom which the curves are plotted will allow other values t0 be used, ifdesired,.without.repeating the calcu-v lations.

It should be emphasized that a, straight length of duct equal to atleast 2L, that is twice the length in which lamps will be placed, isrequired if the disclosed calculations are to be used.

Although a preferred embodiment of my inapproximately uniformlydistributed over the central half ofthe duct.V

2. The method of sterilizing air, comprising positioning elongatedultra-violet generating lamps in a straight portion of anair duct sothat their axes `extend transverse to the line of ,movementofyairtherein and their centers are equally spaced;V the` length `ofsaid duct being twice `that of they array-of lamps in order toefficiently use irradiations therefrom.

3. The methodof sterilizing. air, comprising placing `generators ofbactericidal rays in a straight length of duct distributedv over thecentral half of the Jduct length, in order to make it possible to assume.thattheaverage intensity over said duct length is equal to one-half thevalue at the center ofthe array.

l4. Apparatus for sterilizing air, comprising a duct, ultravioletgenerating lamps disposed in a straight portion of said duct, Lthe axesof said lamps extending transverse to the lineof move-y ment of the air,`said lamps being vspaced so as to be approximately uniformlydistributed over the central half of said straight portion.

5,Apparatus for ysterilizing air comprising a duct havinga straightportion `of a predetermined length and 'ultra-violet generating lampsdisposed in said straight portion with their axes extending transverseto the line of movement of air therein, said lamps being spaced so thatthey are approximately uniformly distributed over Ionly the central halfof said straight portion..

6. .The 4methodof sterilizing air comprising spacing ultra-violet raygenerating lamps in a straight portion'of an air duct so that they areapproximately uniformly distributed over the central half of saidstraight portion.

7. The method of sterilizing air comprising positioning elongatedultra-violet ray generating lamps in a straight portion of an air ductso that their axes extend transverse to the line of movement of airtherein and their centers are substantially equally spaced, the straightportion of said duct being extended'substantial distances beyond theendsof the array of lamps in order to eiciently use'radiationstherefrom.

8. Apparatus for sterilizing air comprising a duct having a straightportion of a predetermined length and ultra-violet ray generating lampsdisposed in said straight portions and spaced so that they areapproximately uniformly distributed over only the central half thereof.

'9. ApparatusV for sterilizing air comprising a duct having a straightportion enlarged in crosssection as compared with the normal ductsection, and ultra-violet ray generating lamps distributed in saidstraight portion with their axes extending transverse to the line ofmovement of air therein, said lamps being arranged as a plurality ofrows extending longitudinally of said straight portion, said rows beingspaced substantially uniformly from vone another with the rows adjacentthe sides ofsaid straight portion positioned from said sides a distanceequal to approximately halfthe row spacing, said straight portion beingextended beyond the ends of said rows a substantial distance and thentapered to the normal Vcross-section of said duct.

EDWARD G. F. ARNO'I'T.

